1. Field of the Invention
The present invention relates to electrical and electronic components, circuits and devices. More specifically, the present invention relates to electrical and electronic components implemented with micro-electro-mechanical (EMS) devices.
2. Description of the Related Art
Various electrical and electronic devices are being implemented in MEMS technology. MEMS technology offers lower losses than conventional implementations of discrete components. Unfortunately, for certain types of components such as variable capacitors, prior MEMS designs have not taken full advantage of the low loss potential of MEMS technology. These prior approaches have been somewhat lossy due to parasitic effects associated with the actuation mechanism of the MEMS devices.
Further, prior MEMS designs have used electrostatic actuation and bimetal strip approaches. Unfortunately, electrostatic attraction is impractical for a variable capacitor implementation and the bimetal approach has been found to be too slow and requires too much power.
Hence, a need existed in the art for an improved variable capacitor design implemented with MEMS technology. This need was met by the teachings of U.S. patent application Ser. No. 10/294,413 entitled MICRO ELECTRO-MECHANICAL SYSTEM DEVICE WITH PIEZOELECTRIC THIN FILM ACTUATOR, filed Nov. 14, 2002 by J. Park et al., the teachings of which are incorporated herein by reference. This application discloses and claims a radio frequency MEMS device with a piezoelectric thin film actuator disposed over a substrate and conductive bumps which serve as spacers. In one embodiment, the device is disclosed as being usable as a tunable capacitor in which the inter-electrode spacing between a conducting path electrode and an RF path electrode is controllably varied by an actuator beam in order to selectively vary the capacitance between the electrodes.
These devices, known as ‘flip-chips’ due to the upside down orientation thereof relative to conventional designs, are typically assembled by screen printing solder paste or conductive epoxy and reflowed at temperatures above 150 degrees Celsius. Unfortunately, these methods cannot be used in applications requiring strict control of the height gap between the flip chip and the substrate that the flip chip is mounted on. The height is not controlled well because the volume of paste or solder has too much variability and a consistent height cannot be achieved. This is particularly problematic with respect to the construction of tunable capacitors.
Most attempts at controlling the height requirement have involved efforts to develop better, more consistent bumps or by controlling the volume of screen-printed material. The bump process is a process that involves many variables that are difficult to control. Bumps are often plated using pulse plating and checked quite often to achieve optimal height. As an alternative, the bumps are lapped afterward to a specific height. This can yield a very uniform bump-to-bump height, but it does not compensate for the variation of volume of the screen printed solder paste or conductive epoxy.
The screen-printed material depends greatly on the tolerances of either the machined stencil or the emulsion on the screen. Laser machined or chemical etch stencils will typically have a tolerance of +/−0.001 mil., which can result in great volume changes if the opening in the stencil is small such as 0.004″. Emulsion screens provide inconsistent volume deposition because of the screen mesh that impedes the flow of material.
Another approach is to assemble the flip chip using thermal compression of the entire chip onto malleable pads. This method yields devices that assemble well at temperatures above 150 degrees Celsius, but the pad height is still difficult to control.
Hence, while the above-referenced application addresses the need in the art, a need remains in the art for an RF MEMS design that is easier to manufacture and an improved associated manufacturing method.